Fabric light control window coverings are known in the art, an illustrative disclosure is U.S. Pat. No. 3,384,519 to Froget. Froget discloses a shade having two parallel mesh fabric sheets with a number of movable vanes disposed between the sheets. The parallel fabric sheets are movable relative to each other in order to control the angle of the blades. This shade is attached to a typical cylindrical roller shade head roller for rolling up the shade and controlling the angle of the blades. Similar disclosures are found in U.S. Pat. Nos. 2,029,675 and 2,140,049 and French Pat. No. 1,309,194. German Pat. No. 382,758 discloses a similar window covering, however, instead of a cylindrical head roller, an elliptical head roller is provided.
As illustrated in the above disclosures, such a window covering is generally a sandwich of three layers which are attached together at various points. As the sandwich rolls around a roller, the layer around the outside must travel a greater distance than the inner layer. The thicker the sandwich is, and particularly the blades, the more pronounced this effect becomes. Thus, in practice with known deployment means, as the window covering rolls up around the head roller, the outer layer must stretch or the inner layer must buckle in order to accommodate the different distances that the two layers follow around the roller. Stretchy fabrics are undesirable because the blades would be unaligned in the lowered position, and if non-stretchy fabrics are used, the inner layer will form buckles, which can cause a permanent wrinkle or crease to develop in the fabric over time. This is because as the window covering is wrapped tightly around the roller there is a constant pressure compressing the buckle in the inner layer between the other layers and against the roller itself. In addition to an unattractive appearance, the buckles also create a high point on the roller which can occur unevenly and cause the window covering to roll up unevenly or skew to one side of the roller.
U.S. Pat. No. 4,344,474 to Berman discloses an insulated shade which includes a number of layers wrapped around a cylindrical head roller. While not a light control shade, the layers of the Berman shade appear to be connected together by bushings. Berman has recognized that the different layers will roll up at different rates and has provided journal plates having slotted holes to retain the bushings. This allows relative sliding between the bushings in an attempt to compensate for the different roll up rates of the different layers.
In roller shades in general it is important that the fabric be fastened to the head roller at an exact right angle to the cut edges of the fabric. This is to insure that the fabric rolls up straight along the head roller without skewing to the left or right. The curtain roller disclosed in U.S. Pat. No. 286,027 to Lobdell is an attempt to solve this problem in typical single sheet roller shades. Two slats are provided, one having tacks extending outward therefrom. The slot with tacks is positioned at the top of the curtain, at a right angle to the longitudinal line of direction of the curtain. The tacks are pressed through the curtain fabric and the second slat is pressed on to the tacks on the opposite side of the curtain. The slats attached to the curtain are slid into a complimentary groove in the roller. This construction has disadvantages in not being self aligning and requiring that the slats be placed very exactly on the curtain fabric.
A further drawback of known development systems for this type of window covering is that they may be rolled up the wrong way. This would result in the blades being folded back over themselves at the point of attachment to the fabric sides giving rise to at least two difficulties. First, the bulk of the window covering when rolled up would be increased. Second, if the blades were initially made without creases at the attachment points, creases would be formed due to the folding over.
A more recent proposal has been made, see copending application Ser. No. 07/921,021 filed Jul. 29, 1992, for rolling and unrolling a layered light control covering device which does not create permanent creases or wrinkles in the layers and also allows for rolling without skewing of the covering device to the left or right. A roller is proposed that has a discontinuous surface formed by outwardly extending lobes or rounded projections which define recesses to receive buckles formed in the inner layer of the covering device as it is rolled on to the roller. Thus, the buckles are not pressed against the roller to form permanent creases or wrinkles. Also the buckles do not create high spots which would cause the covering device to skew to the left or right when rolled onto the roller. The proposed means of attachment for such a covering device to the head roller, which ensures alignment of the covering device with the head roller, includes providing a recess in the roller parallel to the roller axis. The line of attachment between the top vane and one sheer fabric is received in the recess and held in place by a holding means, which includes a triangular channel in the head roller which receives a wedge-shaped filler strip. The recess just described is formed by the vertex of the triangular channel. The wedge-shape of the filler strip cooperates with one of the sheer fabrics and the top vane of the covering device to force the window covering along the line of attachment between the vane and fabric into the triangular channel, thus ensuring proper alignment. A second channel of circular cross section is provided for attaching the opposite sheet fabric and is designed to tension the first vane against the wedge-shaped filler strip, thus, further insuring proper alignment. The circular channel has an opening which is less in the inside diameter of the channel. The second sheer fabric is wrapped around a resilient tubular clamping member which is forced through the opening into the circular channel. Alternatively, the second channel may be V-shaped and the second sheer fabric is secured to a complementary V-strip and inserted in the V-shaped channel such that the second sheer fabric is firmly held between the V-shaped channel and the V-strip.